Process for preconditioning lead or lead-alloy electrodes



United States Patent O PROCESS FOR PRECONDITIONING LEAD OR LEAD-ALLOY ELECTRODES Frank Seth Gaunce, Trail, British Columbia, Canada, as-

signor to Cominco Ltd., Montreal, Quebec, Canada No Drawing. Filed June 1, 1964, Ser. No. 371,801 Claims priority, application Canada, Aug. 8,1963,

881,980; May 5, 1964, 902,071

8 Claims. (Cl. 204-114) ABSTRACT OF THE DISCLOSURE A process for preconditioning a lead or lead-base electrode comprises electrolyzing the electrode in an aqueous electrolyte containing 1-80 grams of fluoride ion per litre.

This invention relates to a process for preconditioning lead or lead base electrodes which are used as anodes in the production of electrolytic zinc.

In the hydrometallurgical process for the recovery of metallic zinc from its ores, the ore is first roasted to convert any sulfides present to oxides. The oxide is then leached with dilute sulfuric acid to form a zinc sulfate solution. The leaching step is followed by one or more purification steps to remove any iron, cadmium, copper or other interfering elements that may be present in the zinc sulfate solution. This is followed by an electrolytic step in which the zinc is recovered as a high purity product.

The recovery of zinc by electrolysis is accomplished by the application of electrical current through insoluble electrodes, causing a decomposition of the aqueous zinc sulfate electrolyte, and the deposition of metallic zinc at the cathode. Oxygen is released at the anode, and sulfuric acid is formed by the combination of hydrogen and sulfate ions.

'ianganese dioxide is used in the purification step of the process to oxidize any ferrous iron to ferric iron and the resultant electrolyte usually contains small amounts of manganese sulfate. This manganese sulfate is oxidized at the anode and precipitates as manganese dioxide.

In the electrolytic step it is customary to use high purity lead or lead alloys such as lead containing a minor amount, e.g. 0.1 to 2.5% of silver as the anode and to use aluminum cathode starting sheets.

Certain difficulties have always been encountered with the use of lead or lead alloy anodes. The anode is the main source of lead in the cathode zinc, and the reaction in the cell at the anode results in the formation of a layer of mud or sludge consisting of PbO PbSO, and MnO When first formed, this anode mud or scale is rather loosely attached to the anode surface and frequently breaks away from the surface of the anode. While the bulk of this mud falls to the bottom of the cell, fine particles suspended in the electrolyte may migrate to the surface of the cathode and be deposited with the zinc.

Over a period of time, usually three to six months, the anode becomes stabilized With a hard, dense adherent layer consisting mainly of PbO PbSO, and MnO formed on the surface of the anode. Once this hard, dense layer has been formed, there is much less lead carried over to the cathode zinc, and there is less time spent on cleaning the cell. The daily average lead in the cathode zinc drops from a high of about 0.006% when using new anodes to a. low of about 0.002% when using aged or stabilized anodes.

The amount of MnO formed at the anode decreases with time and with the formation of the hard, dense layer on the surface of the anode. New anodes precipitate more manganese from solution than anodes that have been in use for some time. Both scale and slude must be removed from the cell at regular intervals, and these intervals vary 3,392,094 Patented July 9, 1968 from about once every ten days when using new anodes in the cell, to once every days or more when using aged or stabilized anodes.

The length of time that an anode can be used depends not only on the amount of corrosion of the lead, but also on the influence of the anode on the character and efficiency of the zinc deposition. After a period of time, usuallythree to four years, there is a gradual decrease in the efliciency of the anode, and it is replaced by a new anode.

The above-discussed process of ageing and stabilizing the lead or lead alloy anodes is usually referred to as conditioning the anodes.

It has now been found that the time required for conditioning a new lead or lead base anode can be reduced from months to less than one week if the anode is preconditioned in an electrolyte containing fluoride ions prior to its use as an anode in the production of electrolytic zinc. In the preconditioning process the anode to be preconditioned is made the anode of an electrolytic cell which also contains a suitable inert cathode such as lead and an electrolyte containing fluoride ions. An electrical current is passed through the cell until the anode is conditioned as evidenced by the formation of a layer of lead dioxide on its surface.

The fluoride ion may be supplied to the preconditioning cell in the form of any soluble fluoride compound, the cation of which does not plate out at the cathode. Particularly suitable sources of the fluoride ion are sodium or potassium fluoride or hydrofluoric acid. Although the concentration of the fluoride ion is not critical with respect to the effectiveness of the preconditioning process, being limited only by considerations of economy, the fluoride ion concentration is of importance for optimum results. A generally suitable concentration of fluoride ion is in the range of from 1 to 80 grams per litre and a preferred concentration is in the range of from 40 to grams of fluoride ion per litre. Concentrations above 80 grams per litre can be used but it is uneconomical to use these high concentrations because of excessive corrosion of the anode by the fluorine. Also, test work has shown that at low concentrations, two grams or less of fluoride ion per litre, satisfactory preconditioning can be attained, but it is uneconomical to use these low concentrations because too long a time or too high a current density is needed for preconditioning.

The actual part played by the fluoride in the preconditioning process is not fully understood. During the electrolysis in the fluoride electrolyte, the surface of the anode becomes coated with an adherent layer of lead dioxide and possibly also lead sulfate if sulfate is present in the elec trolyte. The fluoride does not appear to enter the coating and it is a possibility that the presence of the fluoride ion in the electrolyte in the preconditioning step assists in the rapid corrosion of the anode surface and the formation of the lead dioxide layer in ideal form for use in the electrolysis of zinc sulfate solutions. It has also been found that the electrolyte can contain sulfate ions in amounts up to 400 grams per litre or more with satisfactory results.

The time required to precondition the anodes depends, among other things, on the concentration of the fluoride ion in the electrolyte, lower concentrations requiring longer preconditioning time. Generally, the process will be continued until the first appearance of a uniform, adherent coating over the surface of the anode. This may require as little as 30 minutes or as much as 24 hours or more and the process should be continued until an adherent coating has been formed on the surface of the anode. Generally, the preconditioning will be carried out at a current density in the range of five to one hundred and fifty amperes per square foot of submerged anode surface and preferably at a current density of 45 to amperes per sq. ft.

When the preconditioned anodes have been removed from the preconditioning cell, they are preferably washed with water and inserted into a cell for producing electrolytic zinc or they can be dried and stored for future use.

The invention will now be described in greater detail by reference to the following examples which are intended to be illustrative only, and are not to be construed as limiting the scope of the invention:

EXAMPLE 1 For the purpose of comparison, an electrolysis was conducted in a laboratory cell using anodes which were not preconditioned. The anodes consisted of a lead alloy 7 containing 0.75% by weight of silver and aluminum sheets were used as cathodes with a zinc sulfate electrolyte. The cathode zinc formed during the electrolysis was stripped from the aluminum cathode sheets once every 24 hours and the lead content of the cathode zinc was measured. This information then served as a basis for comparison with the new process of the invention.

The details of the operations were as follows:

Electrolyte:

Zinc 52 gms./litre. H SO 100 gms./litre. Manganese 1.53.0 gms./litre. Cathodes Aluminum starting sheets. Current density 4042 amps per sq. ft. of

electrode area. Electrode spacing 1.5 inches, anode to cath ode. Voltage drop 3.3 to 3.5 volts, anode to cathode. Operating cell temperature 2831 C. Cell cleaned A n o d e mud removed from surface of anodes and bottom of the cell at the end of 4th, 9th and 14th weeks.

The zinc deposit was stripped from the aluminum cathode once every 24 hours.

The lead content of the electrolytic zinc based on weekly averages varied as follows; over a period of 14 weeks:

From the above table, it will be noted that the weekly average of daily figures for lead in the cathode zinc climbed from a low of 0.002% at the end of the first week to a high of 0.0084% at the end of the seventh week. Following clean-up, the weekly average lead in the cathode zinc dropped back to 0.005% and continued at approximately this level.

EXAMPLE 2 Anodes consisting of lead containing 0.75% by weight of silver were preconditioned by electrolysis in an electrolyte containing 10 grams fluorine per litre added as sodium fluoride. The process details were as follows:

Preconditioning Anodes and cathodes Lead-0.75% silver. Current density 21 amps per sq. ft. of

electrode area. Electrodespacing 1.5 inches. 7 Voltage drop 3.5 volts, anode to cathode.

Operating cell temperature 2 8 to 35 C. Conditioning time 21 hours.

At the end of the 21 hour period, the anodes were removed from the preconditioning cell, washed with water and stored in air ready for use in a regular zinc electrolysis cell. :1

These preconditioned anodes were used. asanodes in the electrodeposition of zinc on aluminum cathodes from a zinc sulfate electrolyte under the following operating conditions:

Electrolyte:

Zinc 52 guts/litre. H SO gms./litre. Manganese 1.53.0 gms/litre. Cathodes Aluminum starting sheets. Current density 45-50 amps per sq. ft. of

' electrode area. Electrode spacing 1.5 inches. Voltage drop 3.5 to 3.7 volts, anode to cathode. Operating cell temperature 27 to 40 C. Cell cleaned End of 6th week.

Zinc was stripped from the aluminum cathode once every 24 hours and the amount of leadin the zinc was determined. The electrolysis was continued for av period of 12 weeks and the weekly average lead contents of the zinc were as follows:

Week: Percent Lead 1 .0018

12 s .0005 Overall average .0019

Thus it will be' seen from the above table that the weekly average of the daily figure for lead in the cathode zinc dropped to 0.002% lead atthe end of the second week and to 0.0005% lead at the end of the eighth week of operation, and continued at this low level for the duration of the test, which was. 12 weeks.

EXAMPLE 3' Anodes of lead containing 1.5% by weight of silver were preconditioned by electrolysis in an aqueous e'lectrolyte containing 21 grams of fluorine per litre added as sodium fluoride and 6 grams of sulfuric acid per litre. The preconditioning electrolysis was conducted under the following conditions:

Anodes and cathodes Lead-1.5% silver. Cur-rent density 60 amps per sq. ftxof electrode area. Electrode spacing 1.5 inches. Voltage drop 5.5 volts, anode to I cathode. Operating cell temperature 30 to 35 C. Conditioning time 12 hours.

The anodes were washed with water and allowed to dry.

These preconditioned anodes were used as anodes in the electrodeposition of zinc from a zinc sulfate electrolyte under the following conditions:

Electrolyte:

Zinc 52 gms./litre. H 50 100 gms./litre. Manganese 1.5-3.0 gms./litre. Cathodes Aluminum starting sheets. Current density 45-50 amps per sq. ft. of

electrode area. Electrode spacing 1.5 inches, anode to cath ode.

Operating cell temperature 27 to 40 C. Cell cleaned End of 8th Week.

Zinc was stripped from aluminum cathodes once every 24 hours.

The quantity of lead in the zinc stripped from the aluminum cathodes was determined and a weekly average of the daily lead determinations for a period of weeks was as follows:

Week: Percent Pb 1 .0066

Overall average .0018

From this example it can be seen that sulfate ions can be present in the bath for preconditioning the anodes.

EXAMPLE 4 Anodes of lead containing 0.75% by weight of silver were preconditioned by electrolysis in an aqueous electrolyte containing 41 grams of fluoride ion per litre added as potassium fluoride and 39 gms. of sulfate ion per litre added as sulfuric acid. The preconditioning electrolysis was conducted under the following conditions:

Anodes and cathodes Lead: 0.75% silver.

Current density 50 amps per. sq. ft. of. anode surface.

Electrode spacing 1.5 inches, anode to cathode.

Operating cell temp. 57 C.

Conditioning time 12 hours.

These anodes were Washed with Water and allowed to dry.

These preconditioned anodes were used as anodes in the electrodeposition of zinc from a zinc sulfate electrolyte under the following conditions:

Electrolyte:

Zinc 60 gms./litre.

H 50 110 gms./litre.

Manganese 1 to 2 gms./litre. Cathodes Aluminum starting sheets. Current density 50 amps per sq. ft. of anode area.

Electrode spacing 1.5 inches, anode to cathode. Operating cell temp. -30 C. Cell cleaned End of 5th and 13th week.

Zinc was stripped from the aluminum cathodes once every 24 hours.

The quantity of lead in the zone stripped from the aluminum cathodes was determined and a Weekly average of the daily lead determinations for a period of 20 Weeks was as follows:

EXAMPLE 5 Anodes of lead containing 0.75% by weight of silver were preconditioned by electrolysis in an aqueous elec-v trolyte containing 60 grams of fluoride ion per litre added as potassium fluoride and 45 gms. of sulfate ion per litre added as sulfuric acid. The preconditioning electrolysis Was conducted under the following conditions:

Anodes and cathodes Lead: 0.75 silver.

Current density 50 amps per sq. ft. of anode surface.

Electrode spacing 1.5 inches, anode to cathode.

Operating cell temp. 67 C.

Conditioning time 12 hours.

These anodes were washed with water and allowed to dry.

These preconditioned anodes were used as anodes in the electrodeposition of zinc from zinc sulfate electrolyte under the following conditions:

Electrolyte:

Zinc 6O gms./litre.

H 50 gms./litre.

Manganese 1 to 2 gms./litre Cathodes Aluminum starting sheets. Current density 50 amps per sq. ft. of anode area.

Electrode spacing 1.5 inches, anode to cathode. Operating cell temp. 25-30 C. Cell cleaned End of 6th and 13th week.

Zinc was stripped from the aluminum cathodes once every 24 hours.

The quantity of lead in the zinc stripped from the aluminum cathodes was determined and a Weekly average of the daily lead determinations for a period of 20 weeks was as follows:

Week: Percent Pb Anodes of lead containing 0.75% by weight of silver were preconditioned by electrolysis in an aqueous electrolyte containing 82 grams of fluoride ion per litre added as potassium fluoride and 12 gms. of sulfate ion per litre added as sulfuric acid. The preconditioning electrolysis was conducted under the following conditions:

Anodes and cathodes Lead: 0.75% silver. Current density 80 amps per sq. ft. of anode L surface.

Electrode spacing 1.5 inches, anode to cathode. Operating cell temp. 60 C. Conditioning Time 12 hours.

These anodes were washed with water and allowed to dry.

These preconditioned anodes were used as anodes in the electrodeposition of zinc from a zinc sulfate electrolyte under the following conditions: Electrolyte:

Zinc a. 60 gms./ litre.

H 50 110 gms./ litre.

Manganese l to 2 gms./litre. Cathodes Aluminum starting sheets. Current density 50 amps per sq. ft. of anode area. Electrode spacing 1.5 inches, anode to cathode. Operating cell temp. 25 30 C. Cell cleaned End of 6th and 13th week.

Zinc was stripped from the aluminum cathodes once every 24 hours.

The quantity of lead in the zinc stripped from the aluminum cathodes was determined and a Weekly average of the daily lead determinations for a period of 20 weeks was as follows:

Week: Percent Pb Overall average .0018

EXAMPLE 7 -Anodes of lead (99.99% Pb) were preconditioned by electrolysis in an aqueous electrolyte containing 44 grams of fluoride ion per litre added as potassium fluoride and 17 gms. of sulfate ion per litre added as sulfuric acid.

The preconditioning electrolysis was conducted under the following conditions:

Anodes and cathodes Lead. Current density 50 amps per sq. ft. Electrode spacing 1.5 inches, anode to cathode. Operating cell temp 70 to'85 C. Conditioning time 8 hours.

These anodes were washed with water'and allowed to dry. 1 r

These preconditioned anodes were used as anodes in the electrodeposition of zinc from a zinc sulfate electrolyte under the following conditions:

Electrolyte:

Zinc 50-60 gn1s./ litre.

H SO -110 -gms./litre.

Manganese 1-2 gms./litre. Cathodes Aluminum starting sheets. Current density 50 amps per sq. ft. of anode area. Electrode spacing 1.5 inches anode to cathode. Operating cell temp. 2025 C. Cell cleaned End of 5th week.

Zinc was stripped from the aluminum cathodes once every 24 hours.

The quantity of lead in the zinc stripped from the aluminum cathodes was determined and a weekly average of the daily lead determinations for a period of 8 weeks was as follows:

Week: Percent Pb 1 .0043 2 .0024 3 .0017 4 .0019 5 .0020 6 .0046 7 .0019 8 .0021 Overall average .0026

By way of comparison another test cell was set up using unconditioned pure lead anodes. The operating conditions during the electrodeposition of zinc on aluminum cathodes were the same as those listed above for the conditioned anodes. The weekly averages of the daily lead determinations using these unconditioned lead anodes were as follows:

A comparison of the results obtained in the examples makes it evident that anodes preconditioned in accordance with this invention are most effective in improving the purity of zinc electrodeposited from a zinc sulfate electrolyte. Moreover, such high-grade zinc deposit is obtained within one week of start-up of the process and in addition, very little anode mud is formed, with the result that the cleaning frequency of the cell is greatly reduced. 9

What I claim as my invention is:

1. A process for the electrolytic production of zinc which comprises preconditioning a lead or lead-base anode by providing an aqueous electrolyte containing fluoride ions in a concentration of at least 1 gram of fluoride ion per litre, electrolyzing said electrolyte using said lead or lead-base anode and an inert cathode, recovering the preconditioned anode, and utilizing said pre-conditioned 9 anode as an anode in the electrolytic production of zinc.

2. The process as claimed in claim 1 wherein the concentration of the fluoride ion in the electrolyte is in the range of from 40 to 50 grams of fluoride ion per litre.

3. The process as claimed in claim 1 wherein the leadbase anode contains 0.1 to 2.5% by weight of silver.

4. The process as claimed in claim 1 wherein the electrolyte also contains sulfate ions.

5. A process for the electrolytic production of zinc which comprises preconditioning a lead or lead-base anode by providing an aqueous electrolyte containing 1 to 80 grams of fluoride ion per litre and up to 400 grams of sulfate ion per litre, electrolyzing said electrolyte using said lead or leadbase anode and an inert cathode, recovering the preconditioned anode, and utilizing said preconditioned anode as an anode in the electrolytic production of zinc.

6. The process as claimed in claim 5 wherein the electrolyte is electrolyzed at a current density of five to one hundred and fifty amperes per sq. ft. of submerged anode surface.

7. A process for the electrolytic production of zinc which comprises preconditioning a lead-base anode by providing an aqueous electrolyte containing to 50 grams of fluoride ion per litre, supplied as potassium fluoride, and about 40 grams of sulfate ion per litre, supplied as sulfuric acid, electrolyzing said electrolyte at a current density of to 55 amperes per sq. ft. of submerged anode surface using said lead-base anode and an inert cathode, recovering the preconditioned anode, and utilizing said preconditioned anode as an anode in the electrolytic production of zinc.

3. A process for preconditioning a lead or lead-base electrode intended for use as an anode in the production of electrolytic zinc, which comprises providing an aqueous electrolyte containing 40 to grams of fluoride ion per litre, supplied as potassium fluoride, and about 40 grams of sulfate ion per litre, supplied as sulfuric acid, electrolyzing said electrolyte at a current density of 45 to amperes per square foot of submerged anode surface using said lead or lead-base anode and an inert cathode, and thereafter recovering the preconditioned anode.

References Cited UNITED STATES PATENTS 3/1934 Ewing 2O4l.05 7/1960 Gibson 20457 UNITED STATES P ATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,392,094

July 9, 1968 Frank Seth Gaunce It is hereb certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 38, for 0018" read line 74 0118 for "2011:" read zinc column 5, Signed and sealed this 27th day of May 1969.

(SEAL) Atteat: M Edward M. Fletcher, Jr. Atteating Officer Commissioner 0 Patents 

